To take partial profits on an open position, you can submit an order in the opposite direction using a fraction of the number of contracts of your open position.
If you buy BTC-PERP, the position size is positive. While if you sell BTC-PERP, the position size is denoted as negative, e.g., if you sell 100,000 contracts your position size is -100,000.
To fully close a position, you must reduce the number of contracts to zero. To partially close a position, you just take the opposite trade for a smaller number of contracts.
For example, if you had bought 30,000 BTC-PERP contracts and wanted to take partial profit, you can submit a market order or limit order to sell 15,000 BTC-PERP contracts. Half of the original position will be closed and reduces the position size to 15,000 (=30,000-15,000).
Half of your profits will then be available to withdraw and the other half will still be in the trade as realised profit (which can be used to increase your open position).